Particulate matter and potentially toxic element content in urban ornamental plant species to assess pollutants trapping capacity

Urban environments are usually polluted by anthropogenic activities like traffic, a major source of potentially toxic elements (PTEs), and ornamental plant species may reduce contamination by trapping traffic-related air pollutants in their leaves. The purpose of this study was tested the trapping pollutant capacity of four species commonly used in green areas of Seville city (SW Spain) to better choose species in urban green planning. Composition of particulate matter (PM) obtained from foliar surfaces (sPM) and wax-included (wPM) was determined by EDX-SEM analysis in samples from different city locations. Concentration of different PTEs (Ba, Cd, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn), by microwave induced-plasma optical emission spectroscopy (MP-AES) were also analyzed in unwashed leaves of one of the selected species (Citrus aurantium) since it is the most cultivated species in Seville. Results showed that Nerium oleander was the plant species which trapped best superficial total and coarse PM. This capacity was enhanced by the presence of a waxy-cuticle and by cuticle thickness but not by leaf hairs. The only species unable to trap fine particles was Bougainvillea glabra. The most representative sPM on leaf surfaces from all species was the largest fraction (59-75%), followed by coarse (25-37%) and fine fractions (2.2-4.4%). In the wax PM, 48% of coarse particles were found in Citrus aurantium. Particulate matter deposited on surface foliage in general did not vary seasonally, while the large fraction of wPM in summer was significantly higher than in winter. The seasonal differences also existed in the level of PTE (Cd, Fe, Ni and V) in leaves. This work indicates that the leaf traits should be taken into account to evaluate the pollutants caption capacity, especially when planning of recreational green urban areas. Particulate matter composition was different according to the pollution sources and mostly contained Al, C, Ca, Fe, K, and Mg, but potentially toxic elements such as Si, As, Cr, Cu and Zn just accounted for 0.11-1.95% of the total elemental content.